Obtaining Transport Diffusion Coefficients from Self-diffusion Coefficients in Nanoporous Adsorption Systems

In a continuation of the study of the adsorption and transport properties in a nanoporous adsorption system, transport diffusion coefficients are predicted from the self-diffusion coefficients using the Darken equation, modified for use in the adsorbed phase. We obtain self-diffusion coefficients using equilibrium molecular dynamics (MD) simulations. Adsorption equilibrium data are required in this modified form, which are obtained using grand canonical Monte Carlo (GCMC) simulations. We show that principal component regression provides an elegant and robust method to integrate the data available from the MD and GCMC simulations for the prediction of transport diffusivities. We investigate the effect of the adsorbed phase concentration, mole fraction and temperature on the transport diffusion coefficient thus predicted. We show that the self- and the transport diffusion coefficients decrease with increasing adsorbed phase concentration. We also show that the transport diffusion coefficients decrease with increasing methane mole fraction, in contrast to the behaviour of the self-diffusion coefficients.